The present invention relates to an optical harmonic generator for deriving light (harmonic) with a wavelength which is an integral measure of the wavelength of incident light (fundamental wave) by use of a nonlinear optical effect.
In recent years, with an increase in the availability of small-sized and inexpensive semiconductor lasers, laser-applied devices, such as CD players, video disk devices, and optical disk devices and laser printers as auxiliary devices of a computer, have been developed.
In order to enhance the recording density and operation speed of the laser-applied devices, it is necessary to reduce the wavelength of the laser light used. There is, however, an extreme difficulty in further reducing the wavelength of light emitted from a semiconductor laser.
Conventionally, there has been proposed an optical harmonic generator for converting the wavelength of laser light to a wavelength which is an integral measure of the laser light, by use of a nonlinear optical effect (the phenomenon that the polarization caused in a material by an electric field of light is varied in a nonlinear fashion with an increase in the intensity of the light, due to the interaction between the material and light).
Namely, the conventional device utilizes the phenomenon that a harmonic component of the original light (fundamental wave light) is output due to the nonlinear variation in the polarization when the light is re-emitted by the polarization which is varied in a nonlinear fashion in the material.
However, since the conventional optical harmonic generators based on the above principle are low in conversion efficiency, they have been put to practical use mainly for large-sized lasers such as gas lasers and solid-state lasers by which a sufficiently high light intensity can be obtained.
To apply an optical harmonic generator to a semiconductor laser of which the light intensity is not sufficiently high, however, a high efficiency is required for the optical harmonic generator.
The light intensity of a second harmonic output from an optical harmonic generator capable of generating a second harmonic has the following properties.
(1) The light intensity is proportional to the square of the nonlinear susceptibility of a nonlinear optical crystal and is inversely proportional to the cube of the refractive index thereof.
(2) The light intensity is proportional to the square of the frequency of the fundamental wave light.
(3) The light intensity is proportional to the square of the power density of the fundamental wave light, i.e., to the fourth power of the electric field strength.
(4) The light intensity is proportional to the square of the phase matching length. In some systems, it is proportional to the phase matching length itself.
The light intensity of the harmonic output from an optical harmonic generator capable of generating a third or higher order harmonic has substantially the same properties as described above.
Conventionally, there have been proposed devices in which the electric field strength is increased by using a waveguide structure having an increased phase matching length or a Fabry-Perot resonator structure, as high-efficiency optical harmonic generators.
According to the waveguide type devices, however, the waveguide length must be increased to thereby permit a large phase matching length, making the device extremely large. Further, an extremely high precision of the waveguide structure is required to suppress the attenuation, thus making the manufacture difficult. In addition, since the emitting portion broadens along the waveguide, the profile of the emitted light is deteriorated. Therefore, the devices of this type have the problem of practicability.
On the other hand, according to the resonator type devices, the crystal working precision and temperature setting must be strictly controlled to meet the resonance condition, and thus it is difficult to manufacture the device and to control the temperature during operation. Accordingly, the devices of this type also have the problem of practicability.
An object of this invention is to provide an optical harmonic generator free from the above-described drawbacks.